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Climatic and tectonic controls on Late Triassic to Middle Jurassic sedimenta- tion in northeastern Guangdong Province, South China
Chong-Jin Pang, Zheng-Xiang Li, Yi-Gang Xu, Shu-Nv Wen, Bryan Krapeˇz
PII: S0040-1951(16)30041-5 DOI: doi: 10.1016/j.tecto.2016.03.041 Reference: TECTO 127033
To appear in: Tectonophysics
Received date: 30 December 2015 Revised date: 28 March 2016 Accepted date: 30 March 2016
Please cite this article as: Pang, Chong-Jin, Li, Zheng-Xiang, Xu, Yi-Gang, Wen, Shu- Nv, Krapeˇz, Bryan, Climatic and tectonic controls on Late Triassic to Middle Jurassic sedimentation in northeastern Guangdong Province, South China, Tectonophysics (2016), doi: 10.1016/j.tecto.2016.03.041
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Climatic and tectonic controls on Late Triassic to Middle Jurassic sedimentation in northeastern Guangdong Province, South China
Chong-Jin Panga,b,*, Zheng-Xiang Lib, Yi-Gang Xuc, Shu-Nv Wena, Bryan Krapežd aCollege of Earth Sciences, Guilin University of Technology, Guilin 541004, China bThe Institute for Geoscience Research (TIGeR), ARC Centre of Excellence for Core to Crust
Fluid Systems (CCFS), Department of Applied Geology, Curtin University, GPO Box U1987,
Perth, WA 6845, Australia cState Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry
Chinese Academy of Sciences, Guangzhou 510640, China dDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845,
Australia
* Corresponding author
Present address: College of Earth Sciences, Guilin University of Technology, Guilin 541004, China ACCEPTED MANUSCRIPT
Email: [email protected] (C.J. Pang)
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ABSTRACT
Stratigraphic analyses document climatic and tectonic controls on the filling of a Late
Triassic to early Middle Jurassic (T3-J2) basin that developed on top of a young orogenic belt in southeastern South China. About 2700 metres of Carnian to Bajocian sedimentary rocks are documented in the Meizhou Region. The Carnian to Rhaetian sequence is characterized by deltaic facies that are succeeded by Hettangian fluvial, shallow marine and volcaniclastic facies, and by Sinemurian to early Toarcian interdistributary bay and floodplain facies. The
Late Toarcian to Bajocian sequence comprises proximal alluvial to lacustrine facies that changed upwards to fluvial facies. Fossil assemblages indicate that climatic conditions changed from tropical/subtropical warm humid, to temperate humid, and then to hot arid through the Late Triassic to the Middle Jurassic. Climatically induced changes (e.g., in precipitation, vegetation and erosion) exerted a strong influence on sediment supply, whereas tectonics played a dominant role in the stratigraphic evolution, accommodation generation, sedimentation patterns and volcanism. Tectonostratigraphic analysis shows that the T3-J2 basin was initiated on an orogenic belt during late-stage orogeny, and evolved into shallow- marine and volcanic environments and then back to terrestrial facies during the post-orogenic stage. This was followed by regional uplift and the development of a basin-and-range province. The orderACCEPTED of these events is similar MANUSCRIPT to that of the central Rocky Mountains, western
North America during the Palaeogene. The Mesozoic basin of South China and the Eocene basins of the central Rocky Mountains highlight the importance of subduction-related subsidence above young and broad orogens.
Keywords: Mesozoic, Sedimentation, Sag basin, Climate change, Tectonic control, South
China
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1. Introduction
Shallow-marine platform carbonates and coal-bearing siliciclastic sedimentary sequences developed on the southeastern South China Block during the Late Palaeozoic (Liu and Xu,
1994; Wang, 1985). Those sequences were followed by a period of Late Permian to Early
Mesozoic deformation, metamorphism and magmatism that is known as the Indosinian
Orogeny (Cui and Li, 1983; Hsu et al., 1990; Li and Li, 2007; Li, 1998; Pang et al., 2014;
Ren, 1984; Y.J. Wang et al., 2005; Wang et al., 2013; Zhou et al., 2006). Li and Li (2007) and Li et al. (2012) proposed that the Indosinian orogeny initiated at ca. 280 Ma at the onset of subduction of the Palaeo-Pacific Ocean, proceeding into a period of flat-slab subduction during the ca. 250–190 Ma period. This resulted in the formation of a 1300 km-wide magmatic fold-thrust (i.e., orogenic) belt recording a northwestward younging of magmatism and deformation in southeastern South China. In contrast, Zhou et al. (2006) considered that
250–205 Ma granitoids in the region were generated by collision between the South China
Block and the Indochina Block, followed at ca. 180 Ma by subduction of the Palaeo-Pacific
Ocean. Nonetheless, orogenic movements were interpreted to have ended by ca. 195–190 Ma
(Li and Li, 2007; Zhu et al., 2010).
A Late Triassic to Early Jurassic (T3-J1) basin formed on top of the Indosinian Orogen, evolving from terrestrialACCEPTED to shallow-marine andMANUSCRIPT then back to terrestrial environments (Fig. 1A;
Li and Li, 2007; Liu and Xu, 1994; Pang et al., 2014; Wang, 1985). The T3-J1 basin was interpreted to be a sag basin (Li and Li, 2007) or a post-orogenic basin developed during late stage Indosinian Orogeny (Shu et al., 2009). Deposition in the T3-J1 basin was followed by terrestrial fault-bounded red-bed sedimentation and extensive magmatism throughout eastern
South China from the Middle Jurassic to the Cretaceous (Pang et al., 2014; Shu et al., 2004,
2009; Zhou and Li, 2000; Zhou et al., 2006), in what is commonly referred to as a basin-and- range type province (Gilder et al., 1991; Li and Li, 2007) or an intracontinental extensional
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basin (Shu et al., 2009). New field and geochronological investigations show that the Late
Triassic to Early Jurassic deposits are conformably overlain by the early Middle Jurassic terrestrial succession (named the T3-J2 basin hereafter), which is overlain disconformably by late Middle Jurassic-Cretaceous strata in northeastern Guangdong Province (Figs 1B-C and 2;
GDBGMR, 1988, 1996; Guo et al., 2012; Pang et al., 2014; Shu et al., 2009). Therefore, the
T3–J2 basin spans late-stage orogeny and post-orogenic extension (Pang et al., 2014), such that its stratigraphic architecture and history are key to an understanding of tectonic processes and basin evolution behind active convergent continental margins.
The Indosinian Orogeny records important climatic changes throughout southeastern
South China. For instance, coal-bearing siliciclastic sequences of Late Permian age or of Late
Triassic-Early Jurassic age are overlain by the Middle Jurassic to Cretaceous red-beds.
Palaeontological and geochemical data indicate a climatic change from warm humid during the Late Triassic to hot arid during the Middle Jurassic-Cretaceous (Qian et al., 1987; Xiong et al., 2009; Xu et al., 2010, 2012). However, the role of such climatic change on depositional systems within the T3-J2 basin is poorly constrained.
Late Triassic to early Middle Jurassic (T3-J2) sedimentary rocks are widely distributed throughout southeastern South China, but are particularly well preserved and continuously exposed in the GaosiACCEPTED-Songxi sections, Meizhou MANUSCRIPT region, northeastern Guangdong Province (Fig. 1B-C), despite poor preservation in places due to weathering and vegetation cover.
Therefore, the aims of this paper are to: (1) reconstruct the temporal and spatial evolution of depositional environments in southeastern South China; (2) evaluate the relative importance of climatic and tectonic controls on sedimentation; and (3) provide insights into the link between evolving palaeogeography and lithospheric processes. The work presented here builds on the field and laboratory studies reported in Pang (2014).
2. Geological setting and chronostratigraphy
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The South China Block comprises the Yangtze Block to the northwest and the Cathaysia
Block to the southeast. Collision between the two blocks during the Early Neoproterozoic
(1200-880 Ma) produced the present-day South China Block (Li et al., 2002; Li et al., 2008).
The studied section in Jiaoling County, Meizhou region, northeastern Guangdong Province is located in the south-central Cathaysia Block (Fig. 1B; GDBGMR, 1971). Upper Palaeozoic-
Lower Mesozoic strata comprise Upper Devonian to Lower Carboniferous siliciclastic sedimentary rocks, Upper Carboniferous-Lower Permian carbonates, Middle to Upper
Permian coal-bearing siliciclastic sedimentary rocks, and Lower Triassic carbonates and mudstones (Fig. 1C; GDBGMR, 1971, 1988), which are collectively defined as foreland deposits (Li and Li, 2007). The T3-J2 siliciclastic sedimentary rocks overlie the foreland deposits on an angular unconformity, and are overlain by late Middle Jurassic-Cretaceous volcanic and sedimentary rocks (Gaojiping Group) on a disconformity (Fig. 2; GDBGMR,
1971; Guo et al., 2012). Mesozoic granites are widespread in the region, whereas Mesozoic- aged mafic intrusions, i.e., the ca. 195 Ma Xialan complex, are reported to the west of the study area (Fig. 1B; Zhu et al., 2010).
The Gaosi-Songxi sections crop out along a county road between Gaosi and Baidu (Fig.
1C). Strata were measured in the field to a resolution of 5-10 cm. Because good exposures are generally betweenACCEPTED 5 and 30 metres wide MANUSCRIPT in road cuttings, this study relies primarily on vertical variations of sedimentary facies. Fossils found in the section were identified in the
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences.
Stratigraphic subdivision of T3-J2 strata in southeastern South China has evolved recently with the availability of new age constraints. Four formations (Xiaoping, Jinji, Qiaoyuan and
Zhangping) have previously been defined that span the lower to upper parts of the measured section (Fig. 2). For this study, we redefine the age limits of some of the formations based on new geochronological data (Fig. 2).
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The Xiaoping Formation overlies Lower Triassic or older rocks on an angular unconformity (Fig. 2; GDBGMR, 1988), and is composed of conglomerates, sandstones, shales and mudstones. The chronostratigraphic equivalent of the Xiaoping Formation in northern Guangdong Province is the Genkou Group, which contains Carnian-Rhaetian brackish to shallow-marine bivalves (e.g., Guangdongella-Bakevelloides, Palaeophorus-
Oxytoma and Jiangxiella-Modiolus assemblages) and fossil plants (e.g., Ptilozamites-
Lepidopteris assemblage; GDBGMR, 1988, 1996; Qian et al., 1987; Zhou, 1989; Zhou and
Zhou, 1983). Carnian-Rhaetian bivalves (e.g., Bakevelloids hekiensis, Palaeopharus oblongatus and Pleuromya oblongata) and fossil plants are also present in the Xiaoping
Formation in central Guangdong Province south of the study area (Fig. 2; GDBGMR, 1996), but only fossil plants (the Ptilozamites-Lepidopteris assemblage) were found in the Meizhou region. The Xiaoping Formation is interpreted to be of Carnian to Rhaetian age (ca. 235 to
201 Ma; Fig. 2).
Lower Jurassic shallow-marine siliciclastic sedimentary rocks are generally assigned to the Jinji Formation in Guangdong Province. In the studied section, the Jinji Formation, which conformably overlies the Xiaoping Formation, comprises conglomerates, sandstones and mudstones in its lower to middle parts, and volcanic rocks in its uppermost part (GDBGMR, 1971). Shallow-marineACCEPTED bivalves, such as Cardinia MANUSCRIPT toriyamai Hayami, Hiatella cf. arenicola Terquem present in this Formation are interpreted to be of Hettangian to Sinemurian age
(Chen, 1987; GDBGMR, 1996). A tuff sample from the uppermost part of this formation has a SHRIMP zircon 206Pb/238U age of 199 ± 1.9 Ma (Fig. 3; Appendix Table S1), consistent with the biostratigraphic age. Thus, the Jinji Formation in the Meizhou region was deposited from the Hettangian to early Sinemurian (ca. 201 to 199 Ma; Fig. 2), a much shorter interval than the Jinji Formation in northern and central Guangdong Province (Hettangian to early
Pliensbachian; GDBGMR, 1988, 1996; Qian et al., 1987).
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Conformably overlying the Jinji Formation, the Qiaoyuan Formation chiefly comprises sandstones and mudstones, and is in turn conformably overlain by the Zhangping Formation
(Fig. 2; GDBGMR, 1988; Zhang, 1985). The Qiaoyuan Formation was previously considered to be of late Pliensbachian to Toarcian age (GDBGMR, 1988). Detrital zircon U-Pb ages from a sandstone sample collected in the upper part of the Formation establishes a maximum depositional age of ca. 182 Ma (Pang, 2014). Thus, the Qiaoyuan Formation was probably deposited from the Sinemurian to the early Toarcian in this region (ca. 199 to 182 Ma; Fig. 2).
The Zhangping Formation comprises tuffaceous conglomerates, sandstones, mudstones and siltstones, and is overlain by Gaojiping Group volcaniclastic and volcanic rocks on an unconformity (Fig. 2; GDBGMR, 1988, 1996). LA-ICPMS zircon U-Pb dating indicates that the age of the Gaojiping Group is ca. 168‒ 145 Ma (Guo et al., 2012). Consequently, the
Zhangping Formation is interpreted to have been deposited between the late Toarcian and the
Bajocian (ca. 182 to 168 Ma; Fig. 2).
4. Facies associations and depositional sequences
About 2700 metres of siliciclastic sedimentary rocks were measured in the Gaosi-Songxi sections (Figs 4-6). Thirteen lithofacies are recognized (Table 1) and grouped into ten associations based on interpreted palaeoenvironmental settings. The facies associations for each formation areACCEPTED described in stratigraphic MANUSCRIPT ascending order.
4.1. Xiaoping Formation
The Xiaoping Formation is about 650 m-thick, and comprises two coarsening-upwards rhythms (1st and 2nd units) in the lower half and a set of fining-upwards rhythms (3rd unit) in the upper half. Prodelta, delta-front and delta-plain fluvial facies associations are interpreted for the Xiaoping Formation based on fossils, lithofacies and stacking patterns (Fig.
4).
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4.1.1 Prodelta facies association
This association, identified in the 1st and 2nd units, comprises massive mudstones and siltstones (Mm), carbonaceous mudstones and shales (C), massive sandstones (Sm) and lenticular bedded sandstones (St) (Fig. 4; Table 1). Mudstone beds are from 8 cm- to several
10‘s of cm-thick, whereas sandstone beds vary from 5 to 80 cm-thick. Sandstones range in grain size from fine- to coarse-grained, with lenticular bedding, pyrite nodules and thin coal- seams (about 10 cm-thick). The association is dominated by siltstones, mudstones and carbonaceous mudstones, particularly in the 2nd unit. The thickness of this association is respectively about 115 m and about 130 m in the 1st and the 2nd units. Poorly to well preserved fossil plants are present in these units.
Massive mudstones and siltstones are attributed to suspension deposition with little or no current activity. These mudrock-dominated units are interpreted to have formed in a prodelta environment, where fossil plants may have been introduced by river processes. Absence of marine fossil implies that the deltaic system may have prograded into a lacustrine setting. The lowermost part of the Xiaoping Formation is not exposed, however, it was reported to be composed of conglomerates and sandstones with a thickness of tens of metres (GDBGMR,
1988), and which represent basal fluvial deposits.
4.1.2. Delta-frontACCEPTED facies association MANUSCRIPT
This association is dominated by massive medium-grained sandstones (Sm), along with interbedded massive siltstones (Sm) and mudstones (Mm), and is about 20 m-thick in both the 1st and 2nd units (Fig. 4). Sandstone beds are 20 cm- to 1.5 m-thick, and dark grey to yellowish white in colour. No fossils were found.
This association is interpreted to be the product of a delta-front on the basis of lithofacies and stacking patterns (cf., Bhattacharya, 2010). Coarsening-upward trends, defined by upward change from mud-rich to sand-rich facies, are interpreted to record delta progradation.
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The absence of obvious sedimentary structures indicates that sands settled quickly before being reworked by wave and tidal processes, implying rapid deposition of sand-laden currents in a fluvial-dominated deltaic setting.
4.1.3. Delta-plain fluvial facies association
A package of fining-upwards rhythms (3rd unit) is present in the upper part of the
Xiaoping Formation. For each rhythm, conglomerates and pebbly coarse-grained sandstones are present in the lower part, whereas siltstones, mudstones and shales make up the upper part, showing a fining-upwards pattern (Fig. 4).
Two types of conglomerate are present: (1) massive, sorted, clast-supported cobble- pebble conglomerate (Gcm); and (2) matrix-supported, poorly sorted pebble-sandy conglomerate (Gmg) (Table 1; Fig. 4). Clasts are commonly rounded to sub-rounded, and composed of quartzite, sandstone, chert and granite, ranging from 20 to 10 cm in size in beds that fine upwards (Fig. 7A). Conglomerate beds range from 10‘s of cm to 5 m in thickness, and laterally pinch out in some places. There is no clast imbrication, whereas normal grading is present in Gmg. Basal contacts of conglomerates are generally erosional and sharp, whereas top contacts are always gradational (Fig. 7B). Sandstones vary from fine to pebbly coarse-grained, with planar bedding, horizontal lamination (Sh), planar cross-lamination (Sp), lenticular beddingACCEPTED and trough cross-stratification MANUSCRIPT (St) (Table 1), with shale and mudstone lenses and clasts within bedsets. Sandstone beds are generally 20 cm- to 2 m-thick.
Palaeocurrents, calculated by the EZ-ROSE program (Bass, 2000), indicate that palaeoflow was initially to the southwest but then to the northeast (Fig. 4). The upper part of each upward-fining unit comprises mainly Mm, along with minor horizontal lamination mudstones
(Mh), varying from 10 cm- to 2 m-thick in individual beds. The maximum proportion of mudrocks can reach 60%, implying a meandering fluvial style. Plant fossils are abundant in these mudrocks (Fig. 4).
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The basal conglomerates are interpreted to have been deposited during high discharge in bedload channels (Miall, 1996; Ghazi and Mountney, 2009). Pinch-out conglomerate beds represent gravel lags on channel bases. Poorly sorted conglomerates possibly relate to high river discharges and short transport distances. The channel bases show only slightly curved surfaces without evident disconformities (Fig. 7B), implying that the basin underwent no major episodes of degradation (Hajek, 2012). Lateral pinch-out sandstone beds are interpreted to be point-bar deposits of fluvial channels (Fig. 7B; Ghazi and Mountney, 2009;
Miall, 1996). Planar and horizontal cross-laminations are interpreted to have formed during upper-flow regime in channels, and indicate either a peaked hydrograph or high channel sedimentation rates because otherwise they would have been reworked by the waning flow stage (Fielding, 2006; Foreman et al., 2012). The massive mudstone is interpreted to have been deposited from suspension in floodplain areas (Alves et al., 2003). Thus, a delta-plain fluvial association is suggested for this unit, and the fining-upwards trends could be the product of the lateral migration of river channels (Allen, 1964; Ghazi and Mountney, 2009).
The abundance of plant fossils indicates that water tables were high relative to local topography (Allen et al., 2013).
4.2. Jinji Formation
4.2.1. Fluvial faciesACCEPTED association MANUSCRIPT
This association, recognized in the lower part of the Jinji Formation, comprises mainly
Gmg, Sm, Sp, St, soft-sediment deformed sandstones (ball-and-pillow, Sd) and ripple-marked sandstones (Sr), together with minor Mm, and is 60 m-thick (Fig. 5). Features of channel incision are present in outcrops (Fig. 7C). Conglomerates are pebbly, matrix-supported and poorly sorted, and sharply underlie thin mudstone beds (Fig. 7C-E). Palaeoflow directions interpreted from planar cross-laminations are to the north and northeast (Figs 5 and 7F).
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Fossil plants are present but sparse. This association shows fining-upwards to aggradational trends.
Sharp contacts between pebbly conglomerates and overlying mudstones (Fig. 7E) indicate that either subaerial or subaqueous erosion may have occurred. The ball-and-pillow structure is interpreted to have formed by gravitational loading of sandstones (Mills, 1983; Collinson et al., 2006). The lithofacies and sedimentary structures are consistent with a fluvial origin for this association. In contrast to the fluvial association of the Xiaoping Formation, this unit is characterized by a lower proportion of mudrock and a scarcity of fossil plants (Figs 4-5), implying that either fluvial styles or vegetation cover may have changed.
4.2.2. Shallow-marine facies association
This association is mainly composed of Mm and subsidiary Sm, Sh, Gmg and ripple- marked and horizontal laminated mudstone (Mr and Mh), and is about 70m-thick (Fig. 5).
Mudstone and siltstone beds vary from 3 cm- to several 10‘s tens of cm-thick, and are dark grey, yellowish and brown (Fig. 8). Sheet-like tabular sandstone beds are present and range from 3 to 40 cm-thick. The conglomerate layer is only 30 cm-thick in the studied sections.
Shallow-marine bivalves are abundant in the lower part of this unit (Fig. 5; Appendix Table
S2).
The massive mudstonesACCEPTED are interpreted toMANUSCRIPT have formed from suspension deposition (Miall,
1996). The tabular sandstone sheets were possibly deposited due to flood events, which rapidly spread the sands into the reservoir. A shallow-marine environment is interpreted for the Jinji Formation on the basis of lithofacies, fossils and sedimentary structures (Fig. 8).
Thus, the Jinji Formation documents a third-order marine transgression during the Early
Jurassic in the Meizhou region.
4.2.3. Volcanic facies association
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Basalts, with relict amygdaloidal textures, are about 20 m-thick in the upper part of this
Formation, although severe subtropical weathering has reduced rock exposures along the road cutting to clay minerals (i.e., saprolite). Fresh basalt is found in river sections. The basalts are overlain by tuffs and tuffaceous mudstones, which are about 10 m-thick and generally brown and yellowish in colour. SHRIMP zircon U-Pb dating of one tuff unit indicates that volcanism occurred at ca. 199 Ma (Fig. 3).
4.3. Qiaoyuan Formation
4.3.1 Interdistributary bay facies association
The lower part of the Qiaoyuan Formation is dominated by Mm, with subordinate Sm, showing nested coarsening-upwards rhythms and a total thickness of about 180 m (Fig. 5).
Due to strong weathering, lithofacies types and sedimentary structures are obscured in places.
Fossils were found rarely. However, fossil plants (e.g., Nilssonia sp.) have been documented previously (GDBGMR, 1971). This unit is interpreted to have been deposited in a distributary bay. The massive mudstones and siltstones are interpreted to have been deposited from suspension, whereas the nested coarsening-upwards rhythms are interpreted to represent infilling of the bay by overbank flooding (Elliott, 1974). 4.3.2. FloodplainACCEPTED facies association MANUSCRIPT This association, recognized in the upper part of the Qiaoyuan Formation, comprises Mm and Sm, and is >170 m-thick (Fig. 5). Interbedded sandstones, siltstones, mudstones and silty shales are present in the lowermost part (Figs 5 and 9A). Two 40 cm-thick poor-quality coals are present (Fig. 5). Sandstones are very fine to coarse-grained, and vary from dark grey to white yellowish. Tabular sandstone beds are from 5 cm- to 10‘s cm-thick, showing thickening-upwards trends (Fig. 9B). Mudrocks range from lenticular to very thick (1.5 m), and are dark to dark grey. Fossil plants and shell fragments are preserved scarcely, and
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sedimentary structures are uncommon (Fig. 5). This unit is covered by Late Jurassic volcanic rocks of the Gaojiping Group in the Gaosi section, but it is conformably overlain by the
Zhangping Formation in Songxi town (Zhang, 1985; Fig. 1C).
Coals are characteristic of marsh, swamp or mire deposits of fluvial plain and coastal environments (Aloui et al., 2012; McCabe, 1984). Interbedded siltstone and fine-grained sandstones (Fig. 9A) are interpreted to be fluvial levee deposits. The high proportion of mudrock in this unit suggests that depositional energies were relatively low, which is consistent with the absence of sedimentary structures. The presence of shell fragments indicates that the environment was occasionally influenced by marine processes. This association is interpreted to be the product of a flood plain setting. Nested sandstone sheets
(Fig. 5) may have formed due to overbank flooding or crevassing.
4.4. Zhangping Formation
The Zhangping Formation is dominated by conglomerates, tuffs, tuffaceous siltstones and mudstones in the lower and middle parts, and tuffaceous sandstones, siltstones and mudstones in the upper part, varying from purple reddish, green, brown to white yellowish in colour.
Due to limited exposures, details of the lower and middle parts are take from GDBGMR
(1988), whereas the upper part was measured and studied in detail. ACCEPTED MANUSCRIPT 4.4.1. Proximal alluvial facies association This association mainly comprises conglomerates and fine-grained sandstones, as well as minor tuffs, tuffaceous siltstones and mudstones, and is about 200 m-thick (Fig. 6A;
GDBGMR, 1988). Few sedimentary structures have been documented previously, although fossil plants and fresh-water bivalves were well recorded (GDBGMR, 1988), implying a fresh-water environment, such as river and lake. The coexistence of conglomerates and fresh-
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water bivalve-bearing mudrocks implies that this facies association may have been deposited along a fault-bounded margin of a lacustrine environment, as discussed below.
4.4.2. Lacustrine facies association
This association is dominated by Mm, along with interbedded Gmg and Sm, and is about
750 m-thick (Fig. 6; GDBGMR, 1988). These deposits are variegated, with dark grey, purple reddish, greenish grey, yellowish white and reddish brown colours. Lenticular sandstone bodies are enclosed in mudstones or siltstones. Sandstones are generally highly weathered such that primary sedimentary structures were not observed, although minor cross laminations are present. Fresh-water bivalves have been documented (GDBGMR, 1988).
The dominance of multicoloured mudrocks and tuffaceous rocks indicates that this unit may have formed in a shallow-water lake, with input of volcanic ashes. A lacustrine environment is thus postulated (e.g., GDBGMR, 1988), but more detailed field studies are required to support that interpretation.
4.4.3. Fluvial facies association
This association comprises mainly Mm and Sm, along with minor St, Sp and Sh, showing fining- and coarsening-upwards rhythms, and is about 400 m-thick (Fig. 6B). Sandstones are sorted to well sorted,ACCEPTED and vary from 5 cMANUSCRIPTm- to 2 m-thick in individual beds, whereas mudstones and siltstones are 10‘s of cm- to several m-thick (Fig. 6B). Mudstones and siltstones are multicoloured, being dark grey, purple reddish, yellowish, brown grey and greenish grey (Fig. 9C-F). Sand-filled shrinkage cracks are preserved on the bedding surfaces of purple reddish mudstones (Fig. 9C). The uppermost part of the Zhangping Formation is unconformably overlain by the Late Jurassic Gaojiping Group (Fig. 9F).
A fluvial depositional environment is implied by lithofacies and stacking patterns (Miall,
1996, 2010). The tabular sandstones were probably deposited in bedload channels, whereas
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mudstones and siltstones were deposited on a flood plain. Shrinkage cracks can form not only by desiccation processes on subaerial surfaces but also by synaereses processes in subaqueous environments (Plummer and Gostin, 1981). As well as shrinkage cracks, small sand-filled depressions are present on bedding planes, and these may be raindrop imprints.
Thus, shrinkage cracks in this unit are interpreted to be of desiccation origin. Variegated colours may be due to fluctuations of groundwater during diagenetic processes following deposition: the fluctuations of the water table probably altered redoximorphic (oxidation- reduction) environments in the subsurface. Drab colours (grey, green and brown) were generated largely by the formation of ferrous oxides in a reducing environment, whereas red and yellow colours relate to the formation of ferric oxides (e.g., hematite) in oxidizing environments (Miall, 1996).
4.5. Depositional stacking patterns
Progradational to aggradational stacking patterns are recognized from the lower to upper parts of the Xiaoping Formation, whereas a shift of aggradational to retrogradational and then to progradational stacking patterns is present the Jinji Formation (Figs 4-5). A long-term aggradational to progradational stacking pattern is present in the Qiaoyuan Formation (Fig. 5).
In the Zhangping Formation, the proximal alluvial unit is characterized by progradational to retrogradational ACCEPTEDstacking patterns, whereas MANUSCRIPT the lacustrine unit shows aggradational to progradational stacking patterns (Fig. 6A). In contrast, fluvial deposits in the upper part of the
Zhangping Formation show a progradational to aggradational stacking pattern (Fig. 6B).
To summarize, two long-term progradational-aggradational-retrogradation cycles and a final aggradation to progradational cycle are recorded by the Gaosi-Songxi sections (Fig.10).
5. Palaeontology and palaeoclimate
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About 38 species belonging to 18 genera were found in the Gaosi-Songxi sections during this study. These species, along with 58 other species published by GDBGMR (1971), are listed in Appendix Table S2. Fossil plants are found throughout the T3-J2 strata, whereas fossil bivalves are present only in the J1-2 strata (i.e., the Jinji and Zhangping formations; Fig.
10). Fossil records within each formation are plotted in Appendix Figures S1 and S2.
Detailed descriptions of the species (morphology of leaves, pattern of venation, characteristics of the shelly fossils) have been given previously (Fan et al., 1965; HIGS. 1977;
Chen, 1987; Zhou, 1989). Therefore, only the associated order and relative proportions of fossils are summarized below.
5.1. Fossil assemblage
The floral assemblage of the T3-J2 strata mainly consists of the Cycadophytes, Filicales,
Equisetales, Ginkgoales and Coniferophytes, together with the Pteridosperms and incertae sedis genera. Cycadophytes are represented by the genera Otozamites, Anomozamites,
Pterophyllum, Dictyozamites, Ptilophyllum, Nilssonia, Ctenis, Ctenozamites and Zamites
(Figs 10 and 11A-C). Filicales are characterized by the genera Dictyophyllum, Clathropteris,
Todites, Cladophlebis, Danaeopsis, Marattiopsis, Thaumatopteris, Gleichemites,
Phlebopteris and Coniopteris. Equisetales are represented by the genera Taeniocladopsis,
Neocalamites andACCEPTED Equisetites, whereas Ginkgoales MANUSCRIPT include the genera Sphenobaiera, Baiera,
Phoenicopsis, Sagenopteris and Ginkgoites. Coniferophytes mainly comprise the genera
Podozamites, Elatocladus and Pityophyllum. Pteridosperms are represented by the genera
Ptilozamites, Lepidopteris and Thinnfeldia.
In the lower half of the Xiaoping Formation, the floral assemblage comprises 40 species, belonging to the Cycadophytes (13 taxa, 32.5%), Filicales (8 taxa, 20%), Equisetales (7 taxa,
17.5%), Pteridosperms (2 taxa, 5%), Ginkgoales (4 taxa, 10%), Coniferophytes (4 taxa, 10%;
Table S2; Figs 10 and S1) and incertae sedis (2 taxa, 5%). However, in the upper part of the
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Xiaoping Formation, the floral assemblage is more diverse (48 taxa) (Table S2; Figs 10 and
S1), comprising the Cycadophytes (24 taxa, 50%), Filicales (9 taxa, 19%), Equisetales (5 taxa,
10%), Ginkgoales (2 taxa, 4%), Coniferophytes (5 taxa, 10%), seed (1 taxon, 2%) and incertae sedis species (2 taxa, 4%). This Late Triassic floral assemblage in southeast South
China is characterized by the genera Ptiozamites, Lepidopteris, Neocalamites, Dictyophyllum,
Todites, Cladophlebis, Anomozamites, Pterophyllum, Nilssonia and Sphenobaiera, assigned as the Ptiozamites-Lepidopteris assemblage (Figs 10 and S1; GDBGMR, 1988, 1996; Qian et al., 1987; Zhou and Zhou, 1983). Similar flora of Late Triassic age are documented in
Sweden, eastern Greenland, Poland and Germany (Bonis et al., 2010 and references therein), and in eastern Russia (Volynets and Shorokhova, 2007).
Twenty-eight species of plant fossils are recognized in the Jinji Formation, comprising the
Filicales (10 taxa, 36%), Cycadophytes (5 taxa, 18%), Ginkgoales (4 taxa, 14%),
Coniferophytes (4 taxa, 14%), Equisetales (3 taxa, 10%), Pteridosperms (1 taxon, 3.5%) and incertae sedis (1 taxon, 3.5%; Table S2; Figs 10 and S2). Similarly, 32 species are identified in the Qiaoyuan Formation (Table S2; Figs 10 and S2), comprising the Filicales (11 taxa,
34%), Cycadophytes (10 taxa, 31%), Equisetales (4 taxa, 13%), Coniferophytes (3 taxa, 9%),
Ginkgoales (2 taxa, 6%), Pteridosperms (1 taxon, 3%) and incertae sedis (1 taxon, 3%). Fossils are rare inACCEPTED the Zhangping Formation ;MANUSCRIPT only a few species of the Cycadophytes (2 taxa, Nilssonia sp. and Otozamites sp.), Ginkgoales (2 taxa, Baiera sp. and Sphenobaiera sp.), and
Filicales (1 taxon, Cladophlebis sp.), accompanied by fresh-water bivalves (Tutuella rotunda,
Farganoconcha and Pseudocardinia? sp.) were documented in the lower part (Table S2; Figs
10 and S2; GDBGMR, 1988).
Shallow-marine bivalves (Cardinia tutchei Palmer, Cardinia elongate Dunker, Cardinia toriyamai Hayami, Hiatella cf. arenicola Terquem) were found in mudstones in the Jinji
Formation (Table S2; Figs 10 and 11D-F). Species such as Saxicava arenicola Terquem
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(Hiatella arenicola Terquem), Saxicava arenicola, Mytilus sp., Pleuromya oblonga Fan and
Nuculana sp. are also documented (GDBGMR, 1971). Hiatella cf. arenicola Terquem and
Cardinia toriyamai Hayami are characteristic species of the Jinji Formation.
5.2. Palaeoclimatic conditions
Fossil plants in the lower half of the section are poorly preserved, and may have been subjected to transport that would bias both the richness and the dominance-diversity relationship of the source forest (McElwain et al., 2007). In contrast, fossil plants in the upper half of the section are preserved in overbank mudstones and show little evidence of long- distance transportation, indicating primarily autochthonous burial. Nevertheless, fossil plants from both the lower and the upper parts are dominated by the Cycadophytes (33% and 50%, respectively), subordinated by the Filicales (20% and 19%) and Equisetales (18% and 10%) accompanied by the Coniferophytes (10% and 10%) and Ginkgoales (10% and 4%), indicating similar richness and diversity. These species are commonly preserved in coal- bearing strata in southeast South China (FJBGMR, 1985; GDBGMR, 1988; JXBGMR, 1984;
Qian et al., 1987). Thus, the fossil plants of the Xiaoping Formation can be used to unravel the Late Triassic palaeoclimate in southeastern South China.
Cycadophytes (e.g., Pterophyllum/Nilssonia) generally grew in lowland subtropical to tropical areas withACCEPTED warm moist conditions MANUSCRIPTduring the Mesozoic (Y. D. Wang et al., 2005;
Kustatscher et al., 2010). Most Mesozoic Filicales (ferns), such as the genera Todites,
Cladophlebis, Danaeopsis and Marattiopsis in Eurasia also grew under warm moist conditions in subtropical to tropical regions, although some (e.g., Dictyophyllum and
Clathropteris) mainly occupied moist areas in temperate-warm and subtropical zones (Van
Konijnenburg-Van Cittert, 2002). Modern Equisetales generally grow in tropical to subtropical and temperate zones with humid conditions (Y. D. Wang et al., 2005), and their
Mesozoic species may have occupied similar habitats. Coniferophytes commonly inhabited
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relatively dry areas of upland forest in tropical to subtropical zones, whereas Ginkgoales were mainly restricted to temperate areas during the Mesozoic (Y. D. Wang et al., 2005).
The dominance of Cycadophytes, Filicales and Equisetales in Upper Triassic strata implies that southeast South China was at that time located in a tropical to subtropical zone with warm humid climates (Fig. 10; Qian et al., 1987; Zhou and Zhou, 1983). About 80% of the Cycadophytes disappeared in the Lower Jurassic Jinji Formation, whereas the diversity of
Filicales, Equisetales, Ginkgoales and Coniferophytes are similar to those of the Xiaoping
Formation. Comparable floral assemblages between the Jinji and Qiaoyuan formations indicate that the formations were deposited under similar palaeoclimatic conditions. Floral assemblages and the presence of coals in the Qiaoyuan Formation imply that the climate remained warm humid, but became cooler (temperate) during the Early Jurassic (Zhou, 1995).
There was a change in palaeoclimate from humid to arid during deposition of the Zhangping
Formation and its equivalent, as recorded by fossils, lithofacies and mudrock geochemistry
(Qian et al., 1987; Sun et al., 1995; Xu et al., 2010). Only minor fossil plants are found in the lowermost Zhangping Formation, implying a period of vegetation overturn. On the other hand, the presence of sand-filled desiccation cracks indicates that subaerial exposure occurred under a hot and semiarid to arid climate (Figs 9C and 10). Climate change and the consequent reduction of vegetationACCEPTED, and subaerial exposure, MANUSCRIPT could have caused changes of oxygenation levels in the water column, thereby resulting in the multicolours of sedimentary rocks in the
Zhangping Formation.
6. Discussion
6.1. Controls on sedimentation
Basin-filling units generally result from the interplay between accommodation creation, tectonic uplift and sediment supply (Allen and Allen, 2005; Catuneanu et al., 2009; Muto and
Steel, 1997; Schlager, 1993), as recorded by sedimentary stacking patterns (i.e.,
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progradational, aggradational, retrogradational). Accommodation generation and sediment supply basically depend on allogenic and autogenic factors. Allogenic factors, such as eustasy and basin subsidence, control the generation of accommodation (Allen and Allen, 2005;
Dalrymple, 2010; Gomez-Paccard et al., 2012; Hickson et al., 2005; Huerta et al., 2011), whereas climate changes and tectonic evolution of catchments contribute to sediment supply
(Armitage et al., 2011; Blum and Törnqvist, 2000; Rasmussen, 2004; Smith, 1994).
Autogenic sedimentary processes could also contribute to the stratigraphic architecture, particularly alluvial stratigraphy (Hajek et al., 2012; Stouthamer and Berendsen, 2007).
Correlation between the Gaosi-Songxi succession and the well-studied Daxi-Zhuyuan succession enables an informed reconstruction of T3-J2 stratigraphic evolution (Fig. 12). The
Daxi-Zhuyuan succession in the Xinfeng-Lianping region is characterized by four sedimentation units (collectively named the Lantang Group), showing long-term progradational, aggradational and progradational stacking patterns (Fig. 12). Four stages of basin-filling have been invoked based on stratigraphic analysis of the Daxi-Zhuyuan succession (Fig. 12; Pang et al., 2014). Stage 1 records a retrogradation-progradation cycle that was associated with an increasing but then slightly decreasing subsidence rate. Stage 2 documents a backstepping-aggradation cycle, responding to a complex interplay between moderate subsidenceACCEPTED rates, high sedimentation MANUSCRIPT rates and eustasy. Stage 3 represents a backstepping-progradation period reflecting a change from high sedimentation and high subsidence rates to lower subsidence and sedimentation rates prior to intracontinental uplift.
Stage 4 records continental uplift and the subsequent development of a late Early Jurassic-
Cretaceous basin-and-range province. Following such a framework of basin evolution, we interpret the stacking patterns of individual formations of the Gaosi-Songxi succession in an attempt to exam the influence of eustasy, climate and tectonics.
6.1.1. Stage 1—Carnian to Rhaetian
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This stage corresponds to the Xiaoping Formation in the Gaosi-Songxi sections. It records a progradation-aggradation cycle formed during the Carnian to Rhaetian (Fig. 12). There are two divisions for the stage.
The first division is made up of two coarsening-upward cycles (1st and 2nd cycles) in the lower part of the Xiaoping Formation, which overall show a progradational trend (A/S <1;
Figs 4 and 12). This implies that the overall rate of sediment supply (S) outstripped the rate of accommodation generation (A) (Catuneanu et al., 2009). During this interval, two episodes of deltaic deposition were established (Figs 4 and 12), and that relate to transgression-regression cycles. However, long-term global sea level rose to a maximum at ca. 223 Ma (Fig. 12; Haq et al., 1987), thereby generating a mismatch with the sedimentary response. This indicates that the influence of global sea level on sedimentation patterns was much less than the role of subsidence. Regional stratigraphic correlation suggests that the 1st and 2nd deltaic cycles of the Xiaoping Formation may be coeval with the Hongweikeng and Xiaoshui formations of the Daxi-Zhuyuan succession, which were deposited from ca. 235 to 215 Ma (Fig. 12; Pang et al., 2014). Thus, the estimated overall sedimentation rate of these two sedimentary cycles is relatively low (15 m/myr; Fig. 12). The implication is that the initial basin subsidence rate in the Meizhou region was lower than that of the Xinfeng-Lianping region (Pang et al., 2014).
The second ACCEPTED division, corresponding toMANUSCRIPT the upper part (3rd cycle) of the Xiaoping
Formation, is marked by low sedimentation rates (20 m/myr) but an aggradational stacking of fluvial facies (Figs 4 and 12). This implies that the overall rates of accommodation generation and sediment supply were balanced. This period, however, corresponds to a gradual lowering of global sea level, which reached a minimum at ca. 205 Ma (Fig. 12; Haq et al., 1987). Such a falling sea level would predict gradual progradation of facies tracts. Thus, the aggradational stacking and high sinuosity of the inferred rivers indicate increasing accumulation caused by increases in subsidence. Calculated sedimentation rates during this time interval are relatively
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higher than those of the lower part of the Xiaoping Formation (Fig. 12), indicating higher sediment yields. Increases in sediment supply are consistent with the establishment of a meandering fluvial system (e.g., Constantine et al., 2014). The abundance of fossil plants and coal deposits implies that the Late Triassic climate in South China was predominately characterized by warm humid conditions in a subtropical to tropical zone (Fig. 10; Qian et al.,
1987; Sun et al., 1995; Zhou, 1989; Zhou and Zhou, 1983). It is suggested that vegetation could reduce the erosion rates of the catchment areas and thus decrease sediment production and supply (Blum, 2000; Dalrymple, 2010; Davies and Gibling, 2010; Lopez-Blanco, 1993).
However, in subtropical and tropical regimes, sediment yields can increase because of great precipitation values (>1000 mm/yr), although land plants are common (Blum and Törnqvist,
2000 and reference therein). It thus highlights the climatic controls on increases in sediment supply during these periods. Nevertheless, in order to transport coarse sediments (sands and pebbles) downstream (Figs 4 and 7A-B), a sustained steepened river gradient was necessary
(Foreman et al., 2012), implying gradual uplift of source areas.
6.1.2. Stages 2 to 3—Hettangian to early Toarcian
Stages 2 to 3 defined at the Daxi-Zhuyuan succession correspond to the Jinji and
Qiaoyuan formations of the Gaosi-Songxi succession (Fig. 12). The Jinji Formation is made up of the HettangianACCEPTED to early Sinemurian MANUSCRIPT transgressive-regressive packages marked by relatively high overall sedimentation rates (85.5 m/myr; Figs 5 and 12). Depositional environments shifted from fluvial to shallow-marine, and finally to subaerial volcanic and volcaniclastic during this interval (Fig. 5). It corresponds to a period of increasing global sea level and correlates to a shorter-term sea level increasing-decreasing cycle (Fig. 12; Haq et al.,
1987). The fluvial facies are characterized by predominant coarse-grained sandstones (Fig. 5), indicating that river sinuosity became lower compared to that of the Xiaoping Formation and river styles changing from meandering-type to braided-type. Numerous causes are envisaged
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to trigger the variation of fluvial stratigraphic patterns. At a given subsidence rate, the increasing uplift of source areas can cause the basinward progradation of river and transport of coarse sediments, resulting in proximal deposition. Alternatively, the loss of vegetation possibly related to mass extinction at the Triassic-Jurassic boundary (McElwain et al., 1999) could have resulted in intense weathering, erosion in catchment areas and enhanced sediment flux. This is consistent with the decreasing diversity of Early Jurassic flora, particularly the
Cycadophytes, compared to those of Late Triassic ages in South China (Zhou, 1995; Table
S2). Although new species appeared, the overall coverage of vegetation may have declined significantly. In that case, climatically controlled changes on the loss of vegetation and the increased sediment flux could have accounted for the shift from a meandering channel to a braided morphology (cf., Ward et al., 2000; Foreman et al., 2012), as well as river progradation during the deposition of the basal sandstones of the Jinji Formation.
The change from fluvial facies to shallow-marine facies establishes that there was a rapid backstepping of facies tracts (Figs 5 and 12), suggesting that accommodation creation rapidly outstripped sediment supply, possibly due to increases in subsidence rates. The presence of thick tabular sandstone sheets within the shallow-marine facies records the episodic input of sand (Fig. 8). The time interval for marine deposition in the studied Meizhou region was much shorter thanACCEPTED that in the Xinfeng-Lianping MANUSCRIPT region as recorded in the Daxi-Zhuyuan succession (Fig. 12). Nevertheless, the implication is that the basin became broader and deeper. The subsequent occurrence of subaerial volcanism in the upper part of the Jinji
Formation signifies a regressive cycle (Fig. 8). The low stands in relative sea level and shift from shallow-marine to subaerial volcanic environments recorded a rapid decrease in subsidence rates and basin shallowing in the study region, which was not observed in the
Xinfeng-Lianping region (Fig. 12). The eruption of basalts at ca. 200 Ma coincides with the intrusion of ca. 195-190 Ma gabbros and A-type granites in adjacent regions (Fig. 12; Zhu et
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al., 2010; Pang et al., 2014), thereby identifying a bimodal magmatic cycle. Geochemical and isotopic analyses show that the ca. 195–190 Ma mafic rocks were derived from upwelling asthenospheric mantle (Zhu et al., 2010; Meng et al., 2012), which implies the initiation of anorogenic magmatism and termination of the Indosinian Orogeny. Thus, variations in subsidence rates, and associated magmatism relate to basin tectonics rather than to eustasy.
The Qiaoyuan Formation is characterized by progradation-retrogradation cycles and low sedimentation rates (25.5 m/myr; Fig. 12). Such a low sediment flux is consistent with the establishment of high proportions of mudrock in distributary-bay and floodplain facies (Fig.
5). The presence of flora and coal-seams indicates that the water tables were relatively high
(Fig. 5). Coal seams were formed due to the balance between vegetation production and organic burial (McCabe, 1984), indicating increases in vegetation cover, which in turn could have resulted in decreases in sediment flux. The nested sandstone sheets in this Formation
(Figs 5 and 9A-B) possibly resulted from seasonal flooding events (overbank flooding or crevassing), implying that there were seasonal climatic changes that increased rainfall and erosion of source areas.
The change from subaerial volcanic facies of the Jinji Formation to interdistributary bay facies of the lower part of the Qiaoyuan Formation records a backstepping of facies tracts
(Fig. 5), representingACCEPTED rapid increases in accommodation. MANUSCRIPT Rejuvenated subsidence and gradual global sea level rise could probably have caused the stratigraphic change. During this interval, sea level rose to a maximum at ca. 192 Ma and then fell slightly to a minimum at ca. 187 Ma, followed by a rise to a maximum at ca. 182 Ma (Fig. 12; Haq et al., 1987). However, an overall aggradational-progradational stacking of the interdistributary bay and floodplain facies is inconsistent with the combined long-term global sea level rise and increasing subsidence (Fig. 5), which predict retrogradational facies stacking. In fact, low sedimentation rates and the long-term progradational stacking pattern of the Qiaoyuan Formation imply
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decreasing subsidence rates and subsequent shallowing of the basin to the end of the stage, which is consistent with those processes recorded in the Daxi-Zhuyuan succession (Fig. 12), implying that basin-wide sedimentation was controlled by a uniform lithospheric process.
Nonetheless, it was suggested that basin shallowing and regional uplift could have initiated earlier in the Daxi-Zhuyuan area (i.e., the basin depocentre; Fig. 12) causing a local depositional hiatus (Pang et al., 2014), followed by rejuvenated subsidence and continued sedimentation in the Meizhou region (see below).
6.1.3. Stage 4-late Toarcian-Bajocian
This stage corresponds to the Zhangping Formation, and records a retrogradational- progradational-aggradational cycle formed during the late Toarcian to the Bajocian (Figs 6 and 12). Global sea level began to fall at the same time, to a minimum at ca. 177 Ma, and then rose to a maximum at ca. 168 Ma (Fig. 12; Haq et al., 1987), signifying a mismatch between sedimentary response and global sea level. Rather, rejuvenated subsidence is interpreted to have generated new accommodation. The shift from proximal alluvial to lacustrine, and finally back to fluvial facies (Fig. 6), indicates backstepping but then progradation of facies tracts. As estimated overall sedimentation rates were high (86 m/myr;
Fig. 12), an increasing but later slightly decreasing subsidence rate can thus be envisaged for this stage. As recordedACCEPTED by the Zhangping MANUSCRIPT Formation, deposition was continuous in the
Meizhou and adjacent regions in eastern and northeastern Guangdong Province, whereas a depositional hiatus followed by fluvial facies is recorded in the Xinfeng-Lianping region in northern Guangdong Province, implying a change of depocentre from the Early to early
Middle Jurassic (Fig. 12; Pang et al., 2014). This indicates that the earlier T3-J1 basin was uplifted and broken-up into sub-basins (the initiation of the basin-and-range province; Gilder et al., 1991; Li and Li, 2007; Pang et al., 2014), with rejuvenated rapid subsidence along the coastal region. The deposition of the sub-basins is here interpreted to be terminated by the
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Bajocian (Fig. 12), followed by sedimentation and volcanism throughout eastern South China during the late Middle Jurassic to Cretaceous (Guo et al., 2012).
As shown above, decreases in floral assemblage and formation of reddish deposits and associated desiccation structures record a semi-arid to arid climate for South China during this interval (Figs 10 and 12; Qian et al., 1987; Zhou, 1995). Sedimentary responses to such climate conditions are widely documented in south China (FJBGMR, 1985; GDBGMR, 1988;
HNBGMR, 1984; JXBGMR, 1984) and northwest China (i.e., the Qaidam Basin; Y. D.
Wang et al., 2005). The changes from temperate humid to hot arid climates possibly reflect the Toarcian global warming event (e.g., Dera et al., 2011), or may be linked to regional uplift and extensive contemporaneous magmatism in eastern China. Such a change of climatic conditions could probably be one of the major controls on increases in erosion of source areas and thus sediment supply during this stage (e.g., Lopez-Blanco, 1993).
Collectively, combined tectonic and climatic controls determined the evolution of the basin during this stage.
6.2. Palaeogeographic evolution
On the basis of the T3-J2 stratigraphic records in the Guangdong Province (GDBGMR,
1988; Pang et al., 2014; this work), a refined palaeogeographic evolution pattern is reconstructed (Fig.ACCEPTED 13). Following a depositional MANUSCRIPT hiatus due to the Indosinian Orogeny in South China (Liu and Xu, 1994; Tong and Liu, 2000), a basin initiated on a young orogenic belt during the Late Triassic (during the late stage of the Indosinian Orogeny; Fig. 13A; Li and Li, 2007; Pang et al., 2014). Depositional environments at that time varied from alluvial fan to near-shore lacustrine/swamp and shallow-marine (GDBGMR, 1988) (Fig. 13B). The fluvial and lacustrine facies indicate that the entire basin probably stood above sea level during the Carnian, but the southeastern part of the basin reached sea level during the Norian as reflected by marginal-marine facies in the Daxi-Zhuyuan succession (Figs 12 and 13B;
26 ACCEPTED MANUSCRIPT
Pang et al., 2014). However, predominant fluvial facies of the Rhaetian imply that the basin was again above sea level by that time (Fig. 12). This vertical facies variation indicates a transgression-regression cycle (i.e., basin deepening-shallowing) during the Late Triassic.
Late Triassic drainage systems were dominantly towards the east and the northeast in western
Guangdong Province (GDBGMR, 1988), whereas they were towards the south in northern
Guangdong Province (Fig. 13B; Pang et al., 2014). However, transport directions in northeastern Guangdong Province are more complicated (Fig. 13B) reflecting more variable local topography. The occurrence of coal implies substantial rainfall and vegetation coverage under warm and humid climatic conditions during the Late Triassic. This is consistent with the global Mesozoic palaeogeography, during which the South China Block was located in a tropical to subtropical zone (Golonka, 2007; Li, 1998; Metcalfe, 2006). Overall, the basin in the Guangdong Province underwent varied but relatively slow subsidence during the Late
Triassic (Fig. 12; Pang et al., 2014).
During the Hettangian–Pliensbachian, shallow-marine facies were prevalent in the
Guangdong Province, corresponding to a rejuvenated rapid subsidence of the basin (Fig. 13C;
Pang et al., 2014). Drainage remained towards the south into the basin in northern
Guangdong Province, but dominantly towards the east and northeast in western Guangdong Province (Fig. 1ACCEPTED3C; GDBGMR, 1988; Pang MANUSCRIPT et al., 2014). In northeastern Guangdong Province, drainage was dominantly towards the north and northwest (Fig. 13C), consistent with the Rhaetian drainage, whereas the Pliensbachian to Toarcian drainage is unknown at the present time due to a lack of palaeocurrent data. In this region, thick braided-type fluvial deposits at the base of the Jinji Formation can be attributed to large volumes of sediments supplied from source catchments, which were followed by shallow-marine deposits (Fig. 5).
The occurrence of shallow-marine deposits across almost the entire Guangdong Province implies basin widening and deepening, sinking below sea level again during the Early
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Jurassic (Fig. 13C). In the studied Gaosi-Songxi area, sedimentation of shallow-marine sediments was succeeded by volcanism and then by fluvial deposition (Figs 12 and 13C), reflecting another transgression-regression cycle.
Continuous regional uplift terminated deposition in much of the earlier basin, while localized NE-trending subsidence zones formed in eastern Guangdong Province to receive the Zhangping Formation and its equivalents during the Toarcian to Bajocian (Fig. 13D). The newly formed basins resemble continental rifts, and record the initiation of the basin-and- range province (Fig 13D; Li and Li, 2007). Depositional environments changed from alluvial fan to lacustrine and then to fluvial. Fluvial drainage was dominantly toward the north in northern Guangdong Province, whereas it was dominantly toward the southeast and south in northeastern Guangdong Province (Pang, 2014), forming fresh-water or saline lakes (Fig.
13D). The progradational-aggradational stacking pattern of the Zhangping Formation and its unconformity relationship with the overlying Gaojiping Group imply that the basins were eventually filled and uplifted. These overlying volcanic and sedimentary rocks of the late
Middle Jurassic to Cretaceous age recorded the succeeding stratigraphic history of the basin- and-range province in southeastern South China.
In summary, the studied T3-J2 basin is a hybrid basin that comprises a late-orogenic slow subsidence phaseACCEPTED during the Late Triassic, MANUSCRIPTa post-orogenic rapid subsidence and volcanism phase during the Early Jurassic, and a subsequent late Early to early Middle Jurassic basin- and-range-type extensional phase.
6.3. Geodynamic implications
Stratigraphic data from the Gaosi-Songxi sections indicate that the T3-J2 basin initiated on top of a young magmatic fold-thrust belt, and evolved from terrestrial to shallow-marine, and then back to terrestrial facies from the Late Triassic to Early Jurassic, followed by basin uplift and subsequent development of a basin-and-range province since the Middle Jurassic (Figs
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12-13). Although much attention have been paid to these sedimentary rocks, the mechanism of basin formation is still poorly known.
The T3-J2 basin in southeastern South China is a time equivalent to basins around the
North Atlantic (e.g., the Lusitanian basin and the North Sea). The North Atlantic basins evolved from fluvial to marine environments, and then to basin uplift during the Late
Triassic-Middle Jurassic (Sinclair et al., 1994; Alves et al., 2003, 2009; Tucholke et al., 2007), thereby showing a similar evolutionary pattern to the South China basin. Those North
Atlantic basins formed on a Paleozoic-aged orogen (Caledonian-Hercynian orogenic events;
Færseth, 1996) and originated as half-gräben and gräben due to intracontinental lithospheric extension (Driscoll et al., 1995; Alves et al., 2003, 2009; Tucholke et al., 2007). The timing of basin formation relative to preceding orogeny, and therefore possibly the initiating tectonic driver, is therefore in sharp to the South China basin.
The size and basin-filling pattern of the South China T3-J2 basin is similar to Eocene sedimentary ―ponded basins‖ (also called the ―Eocene Great Basin System‖ in Li (2015)) in the central Rocky Mountains of western North America (Dickinson et al., 1988). Those
Eocene basins sit in the centre of the young Laramide Orogen that has been related to latest
Cretaceous-Paleogene flat subduction of an oceanic plateau on the Farallon plate (Coney and Reynolds, 1977ACCEPTED; Dickinson and Snyder, MANUSCRIPT 1978). The basins underwent nonmarine sedimentation and evolved from fluvial to lacustrine to volcaniclastic facies during the early
Eocene (ca. 53-47 Ma), followed by a basin-wide unconformity (Dickinson et al., 1988;
Smith et al., 2014). A recent study by Smith et al. (2014, 2015) suggested that the basins relate to subsidence above the hinge of the flat-subducted Farallon slab as it rolled back.
However, that model appears to be inconsistent with the broad, roughly circular geometry and the relative position of the basin system in western North America (Li, 2015). Instead, a slab-density-driven subsidence model was favoured by Li (2015). In that latter model, a
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subducting oceanic plateau was gradually transformed from basalt to denser eclogite, the gravitational pull of which caused subsidence of the young and broad orogen.
In southeastern South China, prior to T3-J2 sedimentation, magmatism, deformation and a foreland basin migrated toward the continent interior during the Permo-Triassic time, generating the 1300-km wide Indosinian Orogen (Fig. 13A; Li and Li, 2007). Interpretations of the tectonic driver of the Indosinian Orogeny are controversial. Li and Li (2007) and Li et al. (2012) proposed flat subduction of a Palaeo-Pacific oceanic plateau to account for Early
Mesozoic orogenesis and magmatism in South China (Fig. 13A), whereas others (Shu et al.,
2009; Wang et al., 2013; Zhou et al., 2006) suggested that Mesozoic tectonic events were caused by collision between the Indochina and South China blocks within the Tethyan tectonic domain. The latter interpretation is inconsistent with the regional NE-trending folds and thrusts (Chen, 1999), and a Late Permian to Early Triassic NE-trending foreland basin and its northwestward migration trend during the orogeny (inserted figure in Fig. 13A). The most likely tectonic scenario is that the Indosinian Orogen was a back-arc magmatic fold- thrust belt, the evolution of which relates to low-angle subduction of the Palaeo-Pacific
Ocean.
Initiation of the T3-J2 basin on top of that magmatic fold-thrust belt began in Guangdong Province during lateACCEPTED-stage orogeny (Pang et MANUSCRIPT al., 2014). The basin experienced relatively low subsidence rates and underwent terrestrial to marginal shallow-marine then terrestrial sedimentation during the Late Triassic (Figs 12 and 13B). Detrital zircon grains in sandstones from the upper Triassic Xiaoping Formation are dominated by 430 Ma, 970 Ma and 2530 Ma ages, with subordinate 240 Ma, 360‒ 370 Ma, 520 Ma, 780 Ma, 1200 Ma, 1830 Ma and 3120
Ma ages (Pang, 2014). This indicates that the sediments were mostly recycled from the orogen, i.e., pre-Middle Triassic rocks uplifted and exposed by the Indosinian Orogeny. Late
Triassic subsidence rates is atypical of foreland basins, which initiate with very high rates of
30 ACCEPTED MANUSCRIPT
subsidence (Pang et al., 2014). Gravitational pull of an eclogitizing oceanic flat slab was interpreted to have driven the subsidence of the orogen, and also the initiation of the basin, the same way as the formation of the western American Eocene ―ponded basins‖ (Fig. 13B;
Li, 2015; Li and Li, 2007).
The basin subsided continuously, evolving from terrestrial to shallow-marine facies at the beginning of the Early Jurassic in Guangdong Province (Fig. 13C). Shallow-marine siltstone facies were rapidly succeeded by volcanic facies after 200 Ma in the Meizhou region (Figs 5 and 12). Provenance of sediments during this stage is consistent with a mixture of recycled orogen and synvolcanic sources (Pang, 2014). The occurrence of ca. 195–190 Ma anorogenic mafic magmatism (Fig. 13C; Meng et al., 2012; Pang et al., 2014 and references therein) and the termination of flat-slab progradation by around 200 Ma (Li and Li, 2007) imply that the flat-slab likely started to breakup, thereby allowing asthenospheric upwelling at that time.
Subsequent basin shallowing, changing from shallow-marine to terrestrial facies accompanied by regional uplift, has been interpreted to have been caused by lithospheric rebound following foundering of the flat-slab after ca. 190 Ma (Li and Li, 2007; Meng et al.,
2015), and the return to a normal (high-angle) subduction system along the coastal region
(Fig. 13D; Li et al., 2012). The development of the basin-and-range province may relate to extension due toACCEPTED lithospheric rebound and MANUSCRIPT to back-arc extension in East China since the Middle Jurassic (Gilder et al., 1991; Tian et al., 1992; Zhou and Li, 2000; Ren et al., 2002;
Shu et al., 2009; Li and Li, 2007; Li et al., 2012), thereby defining a protracted tectonic cycle of back-arc compression and extension that began during the Late Palaeozoic.
7. Conclusions
About 2700 metres of sedimentary rocks are preserved in the Gaosi-Songxi succession, which comprises the Xiaoping Formation of Late Triassic age, the Jinji Formation of
31 ACCEPTED MANUSCRIPT
Hettangian to early Sinemurian age, the Qiaoyuan Formation of late Sinemurian to early
Toarcian age, and the Zhangping Formation of late Toarcian to Bajocian age.
Prodelta, delta-front and delta-plain facies associations are identified in the Xiaoping
Formation, and these are succeeded by fluvial, shallow-marine, volcanic, interdistributary bay and floodplain facies associations in the Jinji and Qiaoyuan formations, thereby defining a progradational-retrogradational-progradational cycle. The Zhangping Formation is made up of proximal alluvial, lacustrine and fluvial facies associations, thereby representing a retrogradational-progradational cycle. Fossil plants are abundant and establish that palaeoclimates varied from tropical to subtropical warm humid during the Late Triassic, to temperate humid during the early-Early Jurassic, and to hot arid during the late-Early Jurassic to Cretaceous.
Whereas palaeontological, lithofacies and colour variations appear to reflect changes in climate over time, long-term facies stacking patterns undoubtedly reflect tectonic controls on stratigraphic evolution, sedimentation patterns and volcanism. The basin initiated on a young magmatic fold-thrust belt, evolved into shallow-marine but then back to terrestrial fluvial during the Late Triassic to the Early Jurassic interval. The basin was then uplifted and broke up into smaller-scale basins from the Middle Jurassic. Basin evolution and associated magmatism recordACCEPTED cyclic episodes of intercontinental MANUSCRIPT vertical tectonics that have been related to subduction and later foundering of a Palaeo-Pacific oceanic slab.
Acknowledgements
We are grateful to reviewer Tiago Alves and an anonymous reviewer for their constructive comments and suggestions that improved the quality of the manuscript. We thank Drs. X.-C. Wang, H.-Q. Liu and J. Cao for assisting fieldwork. We thank Professors Z-
T Liao, C-Z Chen, X-W Wu and S-X Wen from the Nanjing Institute of Geology and
Palaeontology, Chinese Academy of Sciences for fossil identification. This work was
32 ACCEPTED MANUSCRIPT
supported by the CAS/SAFEA International Partnership Program for Creative Research
Teams (KZCX2-YW-Q04-06), by the Australian Research Council (grant DP110104799), by the natural Science Foundation of Guangxi Province, China (2015GXNSFCA139016) and by the research grant of State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences (SKLIG-KF-14-06; SKLIG-KF-15-01). This is contribution xxx from the ARC Centre of Excellence for Core to Crust Fluid Systems.
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Figure captions
Fig. 1. Geological maps of the study area. (A) Palaeogeography of the Early Jurassic in southeastern South China (modified after Li and Li, 2007). (B) Distribution of Upper Triassic
a to Middle Jurassic (T3-J2) strata in the Guangdong Province. J1 represents early-Early
b a Jurassic, J1 late-Early Jurassic, J2 early-Middle Jurassic. (C) Geological map of the Gaosi-
Songxi area (modified after 1:200,000 scale geological map―Meixian Sheet (G-50-XXX
III)). Measured sections marked by a-b and c-d. L = Lower; U = Upper; Fm. = Formation; Gr.
= Group.
Fig. 2. Chronostratigraphy of Upper Triassic to Upper Jurassic strata in the study area. Age constraints include SHRIMP and LA-ICPMS zircon U-Pb ages (a-b from Guo et al. (2012); c from Pang (2014); d from this study), bivalves (Cardinia toriyamai Hayami; Hiatella cf. arenicola Terquem) and plant fossils (Ptiozamites chinensis Hsu; Lepidopteris ottonis;
Pterophyllum sinense Li; Pterophyllum ptilum Harris; Nilssonia furcate Chow et Tsao).
Fig. 3. SHRIMP zircon U-Pb age of sample 10GDJL04 collected from the uppermost part of the Jinji Formation. (A) U-Pb concordia plot. Dashed circles represent dating points with large errors, whereasACCEPTED solid circles represent MANUSCRIPT those with small errors. (B) Weighted average age (199.5 ± 1.9 Ma) is calculated for the sample.
Fig. 4. Stratigraphic column of the Xiaoping Formation. vfs = very fine-grained sandstone; fs
= fine-grained sandstone; ms = middle-grained sandstone; cs = coarse-grained sandstone; vcs
= very coarse-grained sandstone; gl = gravel.
Fig. 5. Stratigraphic column of the Jinji and Qiaoyuan formations (see Figure 4 for legends).
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Fig. 6. Stratigraphic column of the Zhangping Formation. (A) Vertical profile and lithology of the Zhangping Formation according to GDBGMR (1988). Long-term stacking patterns are marked. (B) Vertical profile, lithofacies facies association, and stacking pattern in the upper part of the Zhangping Formation measured in this study.
Fig. 7. Outcrop photographs of the Xiaoping (A-B) and Jinji formations (C-F). (A)
Conglomerates in the upper part of Xiaoping Formation. (B) Conglomerate layers sharply overlying mudstone and siltstone. (C) Incised fluvial sandstones underlain by mudstones, shales and siltstones of the Jinji Formation. (D) Conglomerate layer sharply overlain by silty mudstones. (E) Ball-and-pillow structure. (F) Planar cross-lamination in grey sandstone. See
Figs. 3 and 4 for photograph positions.
Fig. 8. Outcrop photographs of mudstone and shale-dominated deposits in the Jinji Formation.
Fig. 9. Outcrop photographs from the Qiaoyuan (A-B) and Zhangping formations (C-F). (A)
Interbedded siltstone and very fine-grained sandstones. (B) Thick medium-grained sandstones and interbedACCEPTEDded shales. (C) Sand MANUSCRIPT-filled cracks in purple-red mudstones (top view). (D) Red, yellow and white mudstones. (E) Dark grey sandstone and mudstones. (F)
Unconformity between the Zhangping Formation and the Gaojiping/Douling Group. See Figs.
4 and 5 for locations.
Fig. 10. Compilation of fossil, palaeoclimate and long-term sedimentary stacking patterns of the Xiaoping and Jinji, Qiaoyuan and Zhangping formations.
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Fig. 11. Photographs of fossil plants and bivalves from the Xiaoping (A-C) and Jinji formations (D-F). (A) Pterophyllum pinnatifidum Harris; (B) Todites denticulatus Krasser;
(C) Anomozamite Loczyi Schenk; (D) Cardinia tutchei Palmer; (E) Imprints of shallow- marine bivalve; (F) Cardinia toriyamai Hayami and Cardinia elongate Punker.
Fig. 12. Stratigraphic correlation and associated external controls in southeastern South
China. (A) Locations for the Gaosi-Songxi and the Daxi-Zhuyuan sections. Dashed boxes mark correlated areas (1 and 2) with Middle Triassic to Middle Jurassic magmatism. (B)
Stratigraphic correlation between the Gaosi-Songxi and the Daxi-Zhuyuan successions.
Stratigraphic column, depositional environment and stacking pattern of the Daxi-Zhuyuan succession after Pang et al. (2014). Palaeoclimates are based on Qian et al. (1987) and this study; global sea level curve after Haq et al. (1987). Ages of magmatic rocks and the geological time scale are from Pang et al. (2014 and references therein).
Fig. 13. Late Triassic to Middle Jurassic palaeogeography and interpreted geodynamics,
Guangdong Province, South China. (A) Depositional hiatus, fold-thrust belt with granitic intrusions during the Ladinian. Inserted regional palaeogeography after Li and Li (2007). (B) Reconstructed CarnianACCEPTED to Rhaetian depositional MANUSCRIPT environments. Drainage directions are marked by arrows with pink, blue and red colours. (C) Hettangian to early Toarcian reconstructed depositional environments. (D) Toarcian to Bajocian basin shallowing and uplift with intrusions, followed by development of the basin-and-range province.
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Table 1 Lithology, facies codes, sedimentary structures and interpreted depositional processes of the Gaosi-Songxi succession.
Facies Lithology Sedimentary structure Geometry Depositional processes codes C Carbonaceous mudstone, Massive Tabular bedsets, laminated Suspension deposition under reduce or shale, to tens of centimetres organic-rich environment coal seam Mm Mudstone, siltstone, shale Massive Tabular to wavy bedsets, Suspension deposition with little or no laminated to several metres current activity Mh Siltstone, silty mudstone Horizontal lamination Tabular bedsets, tens of Suspension setting from low-density centimetres currents Mr Siltstone, silty mudstone Ripple marks Tabular to wavy bedsets, Slow suspension fallout and varied tens of centimetres current strength Md Purple-reddish silty Desiccation cracks, Tabular bedsets, tens of Muddy sediments dried out and mudstone, coarse-grained filled with sandstones centimetres; cracks width developed vertical cracks, filled by sandstone of several centimetres sands later during flood event
Sm Sandstone, fine to coarse- Massive Tabular bedsets, tens of Rapid deposition from sand-laden grained, centimetres currents occasionally pebbly Sh Sandstone, medium-grained Horizontal lamination Tabular bedsets, tens of Traction deposition within upper flow centimetres regime on plane beds Sp Medium-grained sandstone Planar cross- Tabular bedsets, tens of Deposition from straight-crested dunes lamination centimetres or sand waves St Sandstone, medium-coarse Trough cross-bedding Tabular to trough-shape Deposition from lower flow regime grained, and cross-lamination bedsets dunes in channels, and varied current occasionally pebbly strength Sr Sandstone, coarse-grained, Asymmetrical ripple Wavy bedsets, up to 1.5 Migration of 2-D/3-D ripples under occasionally pebbly marks metres unidirectional flows Sd Sandstone, fine to coarse- Ball-and-pillow Wavy to tabular bedsets, Formed by sediments gravity loading grained, occasionally pebbly structures tens of centimetres or dewatering (shale pebbles)
Gcm Clast-supported Massive, un-stratified Tabular beds, tens of Deposition from sheet floods and clast- conglomerate, sorted centimetres to several rich debris flows metres Gmg Matrix-supported Normal grading Tabular beds, tens of Deposition by sandy-matrix debris conglomerate, sandy, poorly centimetres flows sorted to metres The upper-case letters or letter prefixes in the lithofacies codes each designates a textual class, for instance, ‗C‘ represents carbonaceous mudstone and shale, and coal seam; ‗M‘ for mudrock (siltstone, shale, and mudstone); ‗S‘ for sandstone; and ‗G‘ for conglomerate. The lower-case letters or letters suffixes each designates a qualifying structure, where ‗d‘ stands for soft-sedimentary deformation (such as desiccation cracks and ball-and-pillow structures); ‗h‘ for horizontal lamination; ‗m‘ for massive/structureless; ‗p‘ for planar cross- lamination; ‗r‘ for ripple marker; ‗t‘ for trough cross-lamination or cross-bedding; ‗cm‘ for massive but not stratified; and ‗mg‘ for normal grading.
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Highlights
Late Triassic to early Middle Jurassic basin formed over a young orogen; Fluviodeltaic, shallow-marine, volcaniclastic and lacustrine facies identified; Warm humid to hot arid climates documented by fossils and sediment; Climatic changes influenced sediment supply; Flat-slab subduction-related tectonics controlled basin evolution and volcanism
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